Method of making a directly-heated cathode

ABSTRACT

A directly-heated cathode having a very short warm-up time comprising a tubular sheet of refractory metal having an array of apertures therein defined by a grid of intersecting filaments. The filament grid may be integral with spaced end sections of the sheet, the end sections serving as heat-dam and top-cap sections of the cathode. Also included is a method of making such a cathode comprising etching the apertures in a refractory metal sheet.

n United States Patent 1 1 1111 3,860,378

Lee et al. 45 A 2, 1974 [5 METHOD OF MAKING A 7 3,912,743 11/1959 Gerard29/2314 DIRECTLYJ-IEATED CATHODE 3,546,075 12/1970 She etz ct 156/82,795,726 6/1957 Ganswindt et 31.. 313/356 [75] Inv n r R r Ar h Lltitz;Albert 3,404,442 10/1968 Aungst et al 29/2314 Bazarian, Jr., Lancaster,both of Pa.

[73] Assignee: RCA Corporation, Princeton, NJ. primary Examiner Roy Lake[22] Filed: June 7 1972 Assistant Examiner-J. W. Davie Attorney, Agent,or Firm-Glenn H. Bruestle; Irwin [21] Appl. No.: 260,504 M KrittmanRelated US. Application Data Division of Ser. No. 81,449, Oct. 16, 1970,

abandoned.

US. Cl. 29/25.]8, 29/2514, 313/348, 313/356 Int. Cl. HOlj 9/38 Field ofSearch 29/2514, 25.17, 25.18; 156/2, 7, 8; 313/346, 348, 356

References Cited UNITED STATES PATENTS 3/1971 Koshizuka 313/346 [57]ABSTRACT 2 Claims, 6 Drawing Figures PATENTED APR 2 I974 F47 will MmmFig. 2.

EWENTORS Robert ,4. Lee and AGENT 5 Albert Bazarz'amJz A METHOD OFMAKING A DIRECTLY-HEATED CATHODE This is a division, of application Ser.No. 81,449, filed l-l6-70 now abandoned.

BACKGROUND OF THE INVENTION This invention relates to a noveldirectly-heated cathode having a very short warm-up time and to a methodof making the directly-heated cathode.

Directly-heated cathodes are employed in electronbeam tubes, such as theRCA type 8462 power tetrode, when short warm-up times are required. Atypical directly-heated cathode comprises a hollow cylinder made from awoven mesh or screen of fine wire. As is known in the art, the finer thewire of the mesh, the shorter is the warm-up time of the cathode.However, a mesh cylinder made from very-fine wire has little structuralstrength or rigidity and is difficult to make.

A simplified method of making directly-heated cathodes having shortwarm-up times and improved structural properties is described in U.S.Pat. No. 3,404,442, issued to S. E. Aungst et a]. The method comprisesforming a thin-wire mesh, i.e., a filament section, be-

tween a pair of supports mounted in spaced relation on a mandrel made ofa disposable material. The ends of the mesh are bonded to the supports,and the wire strands of the mesh are bonded to one another at theircross-over points. Typically, the bondings are accomplished by platingthe parts with nickel and then firing the assembly to diffuse thenickel.

Cathodes made by this method, however, have several disadvantages. Thinwire meshes present structural problems, so that the cathode warm-uptimes are longer than would be obtained if thinner wires could be used.The various metal bonds are sources of cathode failures. Also, thechoice of mesh-wire metals is limited by the need for nickel plating;e.g., tungsten is not used. The method itself has several disadvantages.Very accurate control is required of the wire-winding, nickel-plating,and final-firing procedures. Also, the filament, heat-dam, and top-capsections of the cathodes must be separately made.

SUMMARY OF THE INVENTION The novel directly-heated cathode comprises atubular sheet of refractory metal having an array of apertures thereindefined by a grid of intersecting filaments formed from a single pieceof refractory metal. A source of electrical current is connected to theends of the sheet to heat the filaments. Preferably, the sheet includesend sections integral (i.e., made from the same piece) with the grid ina central section therebetween. By being formed from a single piece ofrefractory metal, the filaments may be made very thin without sufferingthe structural problems of prior cathodes. The prior bonds at thefilament cross-over points are eliminated as sources of cathode failure.Also, by having end sections integral with the filament grid, the priorbonds between the filament and heat dam sections and between thefilament and top-cap sections can be eliminated as sources of cathodefailure. Thus, the novel cathode has a shorter warm-up time and longeroperating lifetime than have prior directly-heated cathodes.

The method of making the novel directly-heated cathode comprises etchingan array of apertures in a sheet of refractory metal to produce a gridof intersecting filaments, shaping the sheet into a tube, and sealingtogether the adjacent sides of the tube. Means are then attached forconnecting a source of current to the upper and lower ends of the tube.Preferably, the apertures are etched in a central section integral withspaced end sections of the sheet. By etching the apertures in the sheetinstead of winding thin wire to produce a filament section, the priorwire-winding, metalbonding, and final-firing procedures are eliminated.The choice of filament material is no longer limited by the need fornickel plating, so that tungsten and other refractory metals can beused. Also, by etching the apertures in the central section of thesheet, the filament section may be integral with the heat-dam andtop-cap sections of the cathode. Thus, with respect to prior cathodes,directly-heated cathodes made by this method are simpler and less costlyto construct, and have shorter warm-up times and longer operatinglifetimes.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side view, in axialsection, of a cathode assembly comprising the novel directly-heatedcathode;

FIG. 2 is a side view, in axial section, of a subassembly of the cathodeassembly of FIG. 1;

FIG. 3 is an unfolded view of a cylindrical member of the sub-assemblyof FIG. 2;

FIG. 4 is an enlarged view of a portion of the member of FIG. 3;

FIG. 5 is a sectional view through line 5-5 of the sub-assembly of FIG.2; and

FIG. 6 is a side view, in axial section, of another subassembly of thecathode assembly of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The following is an example ofthe novel directlyheated cathode. FIG. 1 shows a novel directly-heatedcathode assembly 21 suitable for use in an electron beam tube such as anRCA type 8462 power tetrode. The cathode assembly 21 comprises twosubassemblies: a filament assembly 23 and a support assembly 25.

The filament assembly 23, shown separately in FIG. 2, comprises acylindrical sheet 27 made of tungsten metal and having a thickness ofthe order of 0.001 inch. The cylindrical sheet 27 comprises a centralsection 29 integral (i.e., made from the same piece) with and locatedbetween a lower end section 31 and an upper end section 33. The centralsection 29, the lower end section 31, and the upper end section 33 serveas filament, heat-dam, and top-cap sections, respectively, for thefilament assembly 23.

For further describing the cylindrical sheet 27, reference is made toFIGS. 3 and 4. FIG. 3, which is an unfolded view of the cylindricalsheet 27, shows a rectangular sheet 35 comprising a central section 37integral with and located between a lower end section 39 and an upperend section 41. The central section 37, lower end section 39, and upperend section 41 of the rectangular sheet 35 correspond to the centralsection 29, lower end section 31, and upper end section 33,respectively, of the cylindrical sheet 27.

The central section 37 has a plurality of smaller apertures 43 therein,shown in detail in FIG. 4. The smaller apertures 43 are substantiallyrhombic in shape, the

rhombuses being uniformly separated by a plurality, i.e., grid, ofintersecting filaments 45. The rhombuses have rounded corners toincrease the effective areas of intersection of the filaments 45. Innormal operation, the filaments 45 have currents flowing along theirlengths. Since these currents add at the intersections of the filaments45, the effective areas of intersection are increased to maintain asubstantially uniform current density throughout the central, i.e.,filament, section 29 of the filament assembly 23. Typically, the widthof each filament 41 is of the order of 0.001 inch and the effectivewidth of each intersection of the filaments 41 is of the order of 0.002inch.

The lower end section 39 of the rectangular sheet 35 has a plurality oflarger apertures 47 therein. The larger apertures 47, which aresubstantially rectangular in shape, serve to decrease the area for heatconduction in the lower end, i.e., heat dam, section 31 (whereby heat iscontained in the central, i.e., filament, section 29) of the filamentassembly 23.

Within the filament assembly 23, the cylindrical sheet 27 is supportedat its lower end section 31 by a metal support cylinder 49 secured tothe inner circumference thereof. As shown in FIG. 2, the supportcylinder 49 has a step-like outer portion or flange upon which thecylindrical sheet 27 rests. The cylindrical sheet 27 is in turn lockedto the support cylinder 49 by a metal locking ring 51, which fits aroundthe outer circumference of the lower end section 31 and also rests uponthe outer flange of the support cylinder 49. The cylindrical sheet 27 issupported at its upper end section 33 by a metal filament cap 53 securedto the inner circumference thereof. As shown in FIGS. 2 and 5, thefilament cap 53 is a disk-shaped member comprising a plurality of spokes55, the purpose of which is discussed below. The cylindrical sheet 27 isfurther supported by a metal alignment ring 57 secured to the innercircumference of the lower end section 31, along an area adjacent thecentral section 29, thereof.

As recited above, the cathode assembly 21 comprises the filamentassembly 23 and a support assembly 25. The support assembly 25, shownseparately in FIG. 6, comprises a metal support tube 59, the lower endof which is secured to a terminal assembly 61. The terminal assembly 61includes a metal first ring 63 secured to the support tube 59, a metalcup-shaped member 65, and an insulating second ring 67 connected to andelectrically separating the first ring 63 and the member 45.

The filament assembly 23 and the support assembly 25 are combined, asshown in FIG. 1, such that the support cylinder 49 and the filament cap53 (of the former) are secured to the cup-shaped member 65 and the upperend of the support tube 59 (of the latter), respectively. To compensatefor any expansion of the cylindrical sheet 27 relative to the supporttube 59, due to differential heating, the filament cap 53 comprises theplurality of spokes 55 cited above. The spokes 55 serve as flexiblemechanical couplers, the absence of which could cause the centralsection 29 of the cylindrical sheet 27 to bow in relation to the supportassembly 25 or otherwise cause the cathode assembly 21 to fail.

For operating the cathode, a source of electrical current (not shown) isconnected to the ends of the cylindrical sheet 27 by means ofa firstelectrical lead 69, attached to the lower end of the support tube 59,and a second electrical lead 71, attached to the support cylinder 49.The lower end of the support tube 59 is electrically connected to theupper end section 33 of the cylindrical sheet 27 by means of thefilament cap 53 secured thereto, and the support cylinder 49 iselectrically connected to the lower end section 31 of the cylindricalsheet 27.

Cathodes as described above have had warm-up times as much as 30-to-50percent shorter than the warm-up times of prior (nickel-plated)directly-heated cathodes. Also, the structural properties (e. g. theshock and vibration characteristics) and, therefore, the operatinglifetimes have been improved over the prior art.

The following is an example of the novel method of making thedirectly-heated cathode described above. As shown in FIG. 3, arectangular sheet 35, made of tungsten metal and having a thickness ofthe order of 0.001 inch, comprises a central section 37 integral withand located between a lower end section 39 and an upper end section 41.A plurality of smaller apertures 43 and a plurality of larger apertures47 are photoetched in the central section 37 and the lower end section39, respectively. The smaller apertures 43 are produced in a grid-likearray as shown in FIG. 4 and described above.

Various techniques for photo-etching a pattern of apertures in a metalsheet are known, particularly in the cathode ray tube art. For example,to produce the apertured rectangular sheet 35 shown in FIG. 3, theentire (tungsten) sheet is first coated with a very thin layer ofphotosensitive material or photoresist, such as KMER or KTFR, both ofwhich are commercially available from Eastman Kodak Company. Thephotoresist is then exposed to arc-lamp or ultra-violet light rays bycontact printing through a negative replica of the pattern of aperturesultimately to be contained in the sheet. The sheet is next washed with asuitable solvent, such as trichloroethylene, so that the unexposedphotoresist is dissolved and washed away while the exposed photoresistremains intact. The sheet is then subjected to a chemical etchant, e.g.,one having a 1:1 volume ratio of hydrofluoric acid-to-hot nitric acid,which attacks the resist-free or uncovered areas, but not the coveredareas, of the sheet.

The photo-etched rectangular sheet 35 is shaped into a cylindrical sheet27 by first placing, i.e., curving, the rectangular sheet 35 into anannular space defined by two concentric ceramic cylinders (not shown).The ceramic cylinders are then inserted within a largerdiametermolybdenum cylinder (not shown). The cylinder-shaping assemblycomprising the molybdenum cylinder, the two ceramic cylinders, and thecurved sheet 35 is then heated at about l,000 C, which temperature issufficient to fix the shape of the curved sheet 35. This heating, incombination with the pressure resulting from the greater expansion ofeach of the inner ceramic cylinders relative to the outer molybdenumcylinder, causes the sheet 35 to take the permanent shape of a cylinder.The adjacent longitudinal edges or sides of the cylindrically-shapedsheet 35 typically have overlapping or abutting portions (not shown),which are then sealed, by spot welding or brazing, to complete thecylindrical sheet 27.

As shown in FIG. 2 and described above, a filament assembly 23 comprisesthe cylindrical sheet 27 secured to each of a metal support cylinder 49,a metal locking ring 51, a metal filament cap 53, and a metal alignmentring 57. Employing a known metal-brazing technique, the metal parts arefirst plated with a 0.0002 to 0.0003- inch layer of copper and then a0.0002 to 0.0003-inch layer of gold. Then the complete filament assembly23 is heated at the flow temperature of the copper-gold combination,about 950 C., to secure the parts.

A separate support assembly 25 is constructed as shown in FIG. 6. Asdescribed above, the support assembly 25 comprises a metal support tube59, the lower end of which is secured to a terminal assembly 61. Theterminal assembly 61 comprises a metal first ring 63, a metal cup-shapedmember 65, and an insulating second ring 67 therebetween. Preferably,the parts of the support assembly 25 are secured by brazing.

The support assembly 25 is inserted within the filament assembly 23 suchthat the cup-shaped member 65 and the upper end of the support tube 59(of the former) contact the support cylinder 49 and the filament cap 53(of the latter), respectively. The cup-shaped member 65 is thenrf-brazed to the support cylinder 49, and the support tube 59 isrf-brazed to the filament cap 53. A first electrical lead 69 is attachedto the lower end of the support tube 59, and a second electrical lead 71is attached to the support cylinder 49. The resulting structure is acompleted directly-heated cathode assembly as shown in FIG. 1.

Cathodes made by the method described above haave exhibited thepreviously-cited improved characteristics. In addition, they have beensimpler and less costly to construct than have been priordirectly-heated cathodes, since the wire-winding, metal-bonding, andfinal-firing steps have been eliminated.

GENERAL CONSIDERATIONS There are various embodiments of the inventionother than those described above. For example, the filament assemblycomprises a tubular sheet which may have a cylindrical, conical, orother shape which is open at both ends. The sheet may be made of arefractory metal (i.e., a material that is difficult to melt) other thantungsten, such as molybdenum, nickel, rhenium, or an alloy thereof. Thesheet may comprise only the filament section of the filament assemblythe heatdam and top-cap sections of which must be secured separately tothe sheet. Also, the outer refractory-metal surface of the filamentsection may be coated with a lower-work-function material to effectelection emission at a lower temperature; typically in the art,carbonate coatings are so employed.

and the shape and size of the apertures defined by these filaments, maybe a function of the grid-to-cathode spacing in the tube. In an RCA type8462 power tetrode, where the grid-to-cathode spacing is typically about0.008 inch, the filament section described above appears (to the grid)as a solid cylindrical emitting surface. For other grid-to-cathodespacings, the width of the intersecting filaments may be other than0.001 inch. The smaller apertures in the filament section may be otherthan rhombic in shape; for example, they may be substantiallyrectangular or circular.

The larger apertures in the heat-dam section of the sheet may bevariously dimensioned and have other than a rectangular shape; also,they may not be required. The supporting means for the tubular sheet maycomprise members other than the filament cap, support cylinder, lockingring, and alignment ring described above; the latter may even beeliminated. Also, the support assembly may have a structure other thanthat shown in FIG. 6.

As indicated above, the filament, heat-dam, and topcap sections of thefilament assembly may be made separately and then secured as required.The smaller and larger apertures may be etched in the filament andheat-dam sections, respectively, using techniques and procedures otherthan those described above. Also, the etching step may follow, ratherthan precede, the tubeshaping step. That is, the flat rectangular sheetmay first be formed into a cylinder and then suitably etched to producethe apertured cylindrical sheet. Various metal-shaping techniques may beemployed in the tube-shaping step. For example, the cylinder may be asection of seamless tubing.

What is claimed is:

1. A method of making a directly-heated cathode having integral filamentand heat-dam sections comprising:

a. etching an array of apertures in a central section of a sheet ofrefractory metal to produce a grid of intersecting filaments, therebyforming said central section into said filament section;

b. etching a plurality of openings in a lower end section of said sheetintegral with said central section, thereby forming said lower endsection into said heat-dam section;

0. shaping said sheet into a tube; and

d. attaching means for connecting a source of current to the upper andlower ends of said tube.

2. The method of claim 1, wherein said apertures are The geometry of thefilament section of the sheet, inphoto-etched in said sheet.

cluding the width of the intersecting filaments thereof

2. The method of claim 1, wherein said apertures are photo-etched insaid sheet.